Mark -- I was using one of my email addresses for system testing of an inquiry from some folks in GMF (or Group_Help, not sure which). In the course of doing so, I deleted and re-created that account a couple of times.
This morning, that address is no longer subscribed to my test group; but when I attempt to send it an invitation, I get an error message that it is "already subscribed." When I post a message to the group, that address does not receive the post; and when I attempt to log in under that address using "email me a link," the email never arrives. When I send a +subscribe message from that address to another group I have, nothing comes back.
I wish I could provide the exact sequence on my end that led to this problem, but I cannot reconstruct that a day later. Sorry. Wondering now if you witnessed this activity and implemented a system-wide ban on the address. Please contact me off-list if you need the group name and email address.